The present invention relates to the field of injection mode-locked lasers and more particularly to a high pulse rate, ring cavity mode-locking Ti-sapphire laser systems.
At many electron accelerators through out the world, there is great interest in developing high duty factor, high average power ( greater than 1W) laser systems with picosecond pulsewidths and repetition rates synchronized to the accelerating cavity RF frequency. Such a laser system can be used to extract electrons from a high-voltage semiconductor-photocathode electron gun during the portion of the RF cycle when electrons are accelerated into the machine. In this way, all of the photoemitted electrons are accelerated and delivered to the ultimate user of the beam; none are wasted as is the case when a DC laser light source is used to create a DC electron beam that must be chopped and bunched prior to injection into the accelerator. This efficient use of the extracted electrons helps prolong the lifetime of the photocathode. Laser systems for this application must emit pulses with gigahertz repetition rates. For example, at the Jefferson National Accelerator Facility, a diode laser system with a pulse repetition rate of 1.497 GHz provides electron beam to three nuclear physics experiment halls. Electron accelerators such as the Mainz Microtron at Mainz, Germany and the MIT Bates Linear Accelerator require pulse repetition rates of 2.445 and 2.856 GHz, respectively. The next generation of electron accelerators may require pulsed laser systems with comparable, if not higher, repetition rates. Other accelerator applications such as Compton polarimetry and gamma ray photon sources may also benefit from the availability of high average power, high repetition rate laser systems.
Diode laser systems meet many of the pulse requirements described above but provide an average output power less than 500 mW. Commercial modelocked Ti-sapphire lasers are high power, tunable light sources, however, their pulse repetition rates are typically less than 100 MHz. These lasers are designed to emit high peak power, femtosecond pulses and this necessitates that the cavity length be long enough to accommodate optics (for example, prism pairs) used to compensate for group velocity dispersion within the Ti-sapphire crystal. In a recent publication, Hoffmann et al. ( Nuclear Instrumentation and Methods A 379 (1996) 15) describe a small, Kerr-lens modelocked Ti-sapphire laser with high average power and a pulse repetition rate of 1.039 GHz. Kerr-lens modelocked lasers, however, sometimes cease pulsing and must be xe2x80x9crestartedxe2x80x9d, a condition unacceptable for reliable photoinjection. Basu et al. (Optical Letters, 14 (1989) 1272) used a gain-switched diode laser to injection seed a Ti-sapphire laser pumped with a Q-switched, frequency-doubled Nd:YAG laser. They obtained 19.4 ps (FWHM) pulses at a rate of 200 MHz within the Q-switched macropulse. In a similar manner, in our paper (Hovater et al, Nuclear Instrumentation and Methods in Physics Research A 418 1998) 280-284) we used a gain-switched diode laser to modelock a slightly modified, commercial standing-wave Ti-sapphire laser. In contrast to the work reported by Basu et al., according to our development, the pulse repetition rate of the modelocked Ti-sapphire laser was varied by setting the diode seed laser repetition rate equal to different multiples of the Ti-sapphire laser cavity fundamental frequency. Pulse repetition rates from 223 MHz to 1.56 GHz were observed with 700 mW average output power for all repetition rates. In this manner, gigahertz repetition rates are obtained with a manageable cavity length (i.e., 67 cm rather than  less than 15 cm). No intracavity modelocking elements are necessary. The gain-switched diode laser serves as a simple, stable master oscillator; it is a trivial matter to obtain gain-switched pulse repetition rates to 4 GHz suggesting that operation at even higher repetition rates may be achieved with this method. The Ti-sapphire laser described here is not actively stabilized yet phase noise measurements indicate timing jitter was relatively low; 2.5 ps at a 223 MHz pulse repetition rate.
In our above-referenced 1998 paper, which is hereby incorporated in its entirety by reference, we described and utilized a linear folded cavity that produced a bi-directional laser oscillation. This effect resulted in xe2x80x9cwastingxe2x80x9d of the Ti-sapphire laser light that exited through the input coupler of the first laser described in that paper. It would be highly more desirable and efficient if all of the Ti-sapphire light were used productively.
As described in the prior art, unidirectional laser oscillation that permits efficient use of substantially all of the Ti-sapphire light produced can be achieved through the use of ring cavity lasers that incorporate one or more intracavity devices. Such intracavity devices are unappealing because they add complexity to the laser design and also add loss to the laser cavity that reduces the useful output power that can be obtained from the laser.
It is therefore an object of the present invention to provide an injection modelocked Ti-sapphire laser system that produces a unidirectional laser oscillation that permits optimum utilization of the produced laser light.
Another object of the present invention is to provide such a laser system that produces unidirectional laser oscillation using a ring cavity without the introduction of any intracavity devices.
According to the present invention there is provided an injection modelocking Ti-sapphire laser system that produces a high pulse rate unidirectional laser oscillation through the application of a ring cavity laser that incorporates no intracavity devices to achieve unidirectional oscillation. An argon-ion or doubled Nd:YVO4 laser preferably serves as the pump laser and a gain-switched diode laser serves as the seed laser. The method of operating such a laser system is also disclosed.